docs: def size(self):
      """The size of the schema. If the underlying data source changes, it may be outdated.
      """
      if self._size is None:
        self._size = 0
        for csv_file in self.files:
          self._size += sum(1 if line else 0 for line in _util.open_local_or_gcs(csv_file, 'r'))
  
      return self._size